(a) Field of the Invention
The invention concerns a process for recovering magnesium and/or nickel sulphite by liquid/liquid extraction.
(b) Description of Prior Art
The reaction of gaseous SO.sub.2 in the absence of air with a mixture of magnesium, iron, nickel and calcium in the form of oxide or silicate, for example asbestos byproducts, leads to the introduction of a soluble magnesium bisulphite. However, during the solubilization of magnesium, the elements (iron, calcium and nickel) are also solubilized. The purification of the solution of magnesium bisulphite or the separation of valuable elements in pure form, such as nickel, by well known methods, is impossible. The precipitation of the elements such as iron, calcium and nickel from this solution by means of NH.sub.4 OH, NH.sub.4 OH+H.sub.2 S or (NH.sub.4).sub.2 S creates problems because the magnesium sulphite also precipitates.
SO.sub.2 is produced in large quantities during the smelting of sulphide minerals of copper, lead and zinc. Moreover, it is believed that every year 20 millions of tons of SO.sub.2 must be removed from coal and heating oil so as to conform to the requirements of the controls of the quality of air. The recovery of SO.sub.2 is not easy. The conversion into sulphuric acid constitutes a solution, as long as a market can be found, which is not always the case. Another approach would be to use SO.sub.2 directly for the extraction of metals (magnesium, iron, nickel) which are present in mineral masses, in the form of silicates. Serpentine (an asbestos byproduct) is an example of magnesium silicate.
The reaction of SO.sub.2 (without oxygen) with a pulp of asbestos byproducts enables to produce magnesium bisulphite. However, during the solubilization of magnesium, the impurities (iron, calcium, nickel) are also solubilized. The precipitation of the impurities of this solution by means of NH.sub.4 OH, NH.sub.4 OH+H.sub.2 S or (NH.sub.4).sub.2 S is possible, but the magnesium sulphite also precipitates. Therefore, this method should not be considered.
Tests for the purification of magnesium by separating the undesirable elements by the known method of precipitating iron and nickel as sulphide in alkaline media have not proved satisfactory because of the simultaneous precipitation of magnesium in the form of sulphite.
Although it is well known to carry out the liquid-liquid extraction of Ni, Fe.sup.2+, Ca and Mg, the selective extraction of Ni.sup.2+, Fe.sup.2+ and Ca of an acid solution of magnesium has never been established.